Convert a decimal number to scientific notation, or build a decimal from a mantissa and exponent.
Standard scientific notation requires exactly one non-zero digit before the decimal point, so the mantissa's absolute value stays in the range 1 ≤ |mantissa| < 10 — this keeps the notation unique for any given number.
Scientific notation always keeps the mantissa between 1 and 10. Engineering notation instead restricts the exponent to multiples of 3 (matching unit prefixes like kilo, mega, milli), which can push the mantissa outside the 1-10 range.
Numbers like 0.0000123 or 6,890,000 are hard to read in plain decimal form. Converting to mantissa × 10^exponent makes the magnitude immediately clear and is the standard way such values are written in math and science.